Wildfires Reversing U.S. Air Quality Progress, Study Finds

A new study published in the journal Science on June 4 reveals that increasing wildfire smoke is undoing decades of air quality improvements across the United States. The research, led by scientists at the University of Iowa and funded by NASA, shows that ground-level ozone — the main ingredient in smog — has reversed its long-term decline and is now rising again as climate change drives more frequent and intense wildfires.

According to AP News, national smog levels dropped by 11% from 2003 to 2015 as federal regulations on power plants, vehicles, and diesel engines took effect. But since 2015, as wildfires have intensified, the nation’s average ground-level ozone has increased by 4%, offsetting roughly five years of regulatory progress.

A Reversal of Hard-Won Gains

“By considering everywhere in the U.S., we actually found an increase in ozone starting from 2015,” said study lead author Weizhi Deng, an atmospheric scientist at the University of Iowa.

Co-author Meng Zhou, a wildfire researcher at the University of Iowa and the University of Maryland, Baltimore County, put it more bluntly: “For the last 20 years, by regulations, we keep decreasing the emissions [for human-caused smog-inducing chemicals]. However, because of wildfires, that is actually from natural hazards, all those kinds of effort were wiped out.”

The study, published in Science, found that ozone levels that had been falling by about 0.65 parts per billion (ppb) per year before 2015 have since risen by about 0.13 ppb annually. In the Midwest specifically, wildfires erased approximately 5.3 years’ worth of ozone-control gains.

How Wildfires Create Smog

Wildfires do not emit ozone directly. Instead, they release precursor chemicals — including carbon monoxide, nitrogen oxides, and volatile organic compounds — that react with sunlight and other pollutants to form ground-level ozone. These gases can travel hundreds or even thousands of miles downwind, meaning smoke from Canadian wildfires can create smog in the U.S. Midwest and East Coast.

“People in the Midwest may think fires burning far away will not affect them,” said study corresponding author Jun Wang, an atmospheric scientist at the University of Iowa. “But once wildfire pollution is in the air, it can move across regions. Pollution from one place can affect air quality in another.”

A Growing Public Health Toll

The health consequences are significant. The study estimates an additional 318 premature deaths per year in the U.S. from ozone-related causes since 2013, with the post-2013 average 46% higher than the previous decade. Researchers calculated these figures using average lifespan, ozone exposure estimates, and population density.

From 2022 to 2024, wildfires exposed an additional 43 million Americans to ozone levels exceeding current federal air quality standards. As NASA Science reported, the 2023 Canadian wildfires — which burned twice as much land as the previous record — showed how widely these risks can spread, with smoke-driven ozone increases stretching across the Midwest and into parts of the Northeast and South.

“Higher daily ozone concentrations can increase asthma attacks, hospital admissions, and mortality,” said Kristie Ebi, a public health and climate scientist at the University of Washington. “It’s not quite as deadly as tiny particles, but it’s still a very important pollutant, which is why it’s regulated.”

AI-Powered Methodology

To track these changes, researchers used deep learning — a form of artificial intelligence — to combine data from approximately 1,000 ground-based air quality stations with satellite observations, weather data, and atmospheric models. This created a first-of-its-kind dataset estimating daily surface ozone from 2003 to 2024 on a kilometer-by-kilometer grid across the contiguous United States.

The study highlights a critical gap in monitoring: the EPA’s ground monitoring stations cover only 2% of the continental U.S., mostly in urban areas. “Experts have long called for expanding the air pollution monitoring network to improve research on wildfire smoke exposure and provide the data needed to better protect public health,” said Teresa Feo, research director at Megafire Action.

Climate Change as a Driving Force

Climate change is amplifying the problem. A 2023 study found that climate change increased the intensity of Canada’s 2023 fire season by at least 50% and doubled the chances of the drier, hotter weather conditions needed for such fires. The average U.S. land burned annually is now 9% higher than the 2003-2014 average.

Rutgers atmospheric scientist Lixu Jin, who was not part of the study, noted that “human-caused climate change is an important contributor, because it increases hot, dry fire-weather conditions in many regions.”

What Comes Next

The study emerges amid political controversy over ozone standards. Both the Biden and Trump administrations have delayed or weakened proposed tightening of regulations. Former EPA Administrator Gina McCarthy criticized the current administration’s approach: “Wildfires cause death and destruction, but the greatest danger may come from smoke and extreme heat increasing the ozone that harms people’s health. So the big question is, when are we going to stop the nonsense from this administration to burn more and more ‘beautiful’ fossil fuels?”

Looking ahead, the findings add to a growing body of research on wildfire smoke’s health impacts. A 2025 study in Nature calculated that 2023 Canadian wildfire smoke killed 82,100 people globally (33,000 in the U.S.) due to particle pollution. Another study projected that by 2050, wildfire smoke would kill more than 70,000 people in the U.S. annually at current rates of warming.

NASA’s John Haynes emphasized the value of satellite monitoring: “NASA Earth observations, along with ground monitoring networks, help reveal air quality risks from wildfires that can cross state lines, giving air quality managers better decision-making information as wildfire smoke affects more communities.”

As climate change continues to intensify wildfire seasons across North America, the study underscores a sobering reality: the clean air gains of recent decades are increasingly fragile, and protecting them will require both aggressive emissions reductions and expanded monitoring infrastructure.